1. Field of the Invention
The present invention relates to opaque, laminated and stretched products composed chiefly of a polypropylene. The present invention is concerned more specifically with opaque, pearlescent and heat-sealable laminated and stretched products by laminating together an opacified, stretched layer comprising a composition in which a specific petroleum resin having a high softening point is blended with a crystalline polypropylene and a layer obtained from a crystalline propylene copolymer having a low-melting point. In the present disclosure, the term "product(s)" means sheet, film, yarn or filaments obtained by slitting such film as above, and molded products of sheet, film, etc.
2. Statement of the Prior Art
In general, pearlescent or opaque products are widely used as general-purpose packaging materials, ornamental materials, synthetic paper or transcription printing paper in the form of uniaxially or biaxially stretched films, or as packing materials, weaving yarns, etc. in the form of tapes ribbons or filaments.
As heretofore known in the art, imparting pearlescence or opaqueness to products has been achieved by (1) adding to polypropylene, lustering materials such as white pigments, extender pigments or pearlescent and others, (2) providing a large number of foams in products by adding foaming agents to polypropylene during molding, (3) adding a large amount of inorganic fillers to polypropylene when molded and, then, stretching them to provide voids therein--see Japanese Patent Publication No 63(1988)-24532 and Japanese Patent Laid-Open No. 63(1988)-117043--and (4) treating molded products with solvents or chemicals to make them dull.
These techniques, however, have the following drawbacks. That is, problems with the first technique are that it is difficult to obtain pearlescence with white pigments and extender pigments, and the pearlescent is very costly and moreover has to be added to polypropylene in large quantities. With the second technique, it is difficult to provide uniform and fine foams in thin products such as films, because the foams have a tend to become large in size. A problem with the 3rd technique is that, since the inorganic fillers are added to polypropylene in large quantities, there is a great deal of drop in fluidity during extrusion or occurrence of clogging of a screen pack. Another problem is that the inorganic fillers tend to absorb moisture, causing poor dispersion of the inorganic filler and generation of foams in the products, and the replacement of polymers in the molding-machine takes much time. Yet another problem is that the product becomes ill-lustered and roughened on its surface. A problem with the 4th technique is that it has to be carried out in the step of post-treatment and needs an additional step of removing the solvents and chemicals, suffering from a disadvantage in terms of the equipment and expense. Another problem is that the product has a dull touch and is ill-lustered.
In search of a method free from such problems of the prior art and capable of making opaque or pearlescent products easily, the present inventors have already found that if polypropylene, to which a specific amount of a petroleum resin having a specific high softening point is added, is stretched under specific conditions, it is then possible to obtain opaque, pearlescent and light-weight stretched products in which numerous fine foams are generated and which are superior in productivity to conventional ones. For that findings, the present inventors filed a patent application (Japanese Patent Application No. 1(1989)-74919). However, it has now been found that such a method is disadvantageous in that when the product is used for packaging, it loses its fine foams due to re-melting caused by heating for heat sealing during bag-making and so loses its opaqueness.
As a result of intensive studies made to overcome the drawback of the method as mentioned just above and obtain opaque and pearlescent stretched products which do not lose its opaqueness during heat sealing, it has been found that the desired stretched products can be obtained by laminating a layer obtained from a crystalline propylene copolymer having a specific low-melting point to a stretched layer obtained from a composition in which a crystalline polypropylene is blended with a specific amount of a petroleum resin having a specific high-softening point, or, if required, by laminating a crystalline propylene copolymer having a specific low-melting point to a unstretched layer obtained from the above described composition followed by stretching.